The present invention is generally directed to adhesives and more specifically to pressure sensitive adhesives such as are used in self-adhesive labels and tapes. Pressure sensitive adhesives include general purpose pressure sensitive adhesives and removable pressure sensitive adhesives and typically comprise styrene-butadiene rubber (SBR) or acrylic copolymers. In particular, the invention is directed to the use of starches including enzyme thinned starches in combination with styrene-butadiene rubber or acrylic copolymers as components of pressure sensitive adhesives.
The use of carbohydrates in adhesive formulations is well known. Since ancient times, starches and natural gums have been used as adhesives; current applications include xe2x80x9clibrary paste,xe2x80x9d photograph and print mounting adhesives, and wallpaper adhesives. They offer the advantages of being derived from renewable resources, are basically non-toxic, and are benign in the environment. While such adhesives serve adequately in their traditional applications, they are relatively inconvenient to use due to their slow-drying solvent (water), which may cause damage to the substrate. Their applications are also limited due to their lack of inherent tackiness, limited strength and lack of flexibility.
While synthetic polymers are of more recent origin, they have been used in adhesive formulations for nearly a century. They offer many advantages over the natural carbohydrate adhesives. There exists a wide variety of chemically-different structures which can be specifically tailored to provide a balance of properties such as strength, flexibility, resistance to heat, cold, and chemicals, and to accommodate a wide variety of application techniques. Their disadvantages include the fact that they are usually petroleum-based products and, thus, are derived from a non-renewable resource. Moreover, they often include solvents and other components that are not benign in the environment, and they may interfere with the recycling of paper and other materials.
While combinations of starch and synthetic SBR latices are well known in the adhesive art, their use as components of peelable pressure sensitive adhesives has been problematic. By peelable pressure sensitive adhesive is meant an adhesive which provides reliable but temporary adhesion. Further, by peelable pressure sensitive adhesive is also meant that the adhesive will maintain its peelability over time periods of weeks or months and not become irreversibly bound to a substrate. In particular, it has been extremely difficult for the art to formulate compositions that provide reliable adhesive strength with subsequent peelability. One solution to this problem is presented by Sakai, U.S. Pat. No. 5,190,818 which describes an adhesive composition comprising an aqueous latex emulsion in combination with starch particles and fine particles of silica gel having particle sizes smaller than the average particle diameter of the starch particles. Nevertheless, there remains a desire in the art for alternative pressure sensitive adhesive compositions utilizing SBR or acrylic latices in combination with significant amounts of a renewable carbohydrate component, such as starch, but not requiring the use of a relatively expensive component such as silica gel.
The present invention provides compositions relatively benign to the environment, that are easily cleaned and removed from equipment and substrates using non-hazardous solvent and are less likely to interfere with the recycling of paper and plastic materials.
The present invention is directed to the combination of a thinned derivatized starch with a synthetic polymer latex in producing a pressure sensitive adhesive. According to a preferred aspect of the invention, the pressure sensitive adhesive is peelable. While the thinned derivatized starch may be thinned according to any of a number of means it is preferably an enzyme thinned starch. The thinned starch preferably has an average molecular weight of from 360 to 50,000 daltons with a molecular weight of greater than 1,134 daltons being preferred.
Preferred thinned derivatized starches according to the invention are those in which the thinned derivatized starch is substituted with a member selected from the group consisting of alkyl, hydroxyalkyl, allyl, aryl, arylalkyl, carbonyl, carboxylic acid, phosphate, and quartenary amine groups with hydroxyalkyl and particularly hydroxyethyl and hydroxypropyl modification being preferred.
While a wide variety of synthetic polymer latices are useful according to the invention those having a glass transition temperature of less than 0xc2x0 C. are preferred with those of less than xe2x88x9220xc2x0 C. being particularly preferred. Particularly useful synthetic latices include those selected from the group consisting of a styrene-butadiene copolymer latex, a carboxylated styrene-butadiene copolymer latex, an acrylic copolymer latex, a styrene-acrylic copolymer latex, and a vinyl acetate-acrylic copolymer latex.
According to one aspect of the invention the thinned derivatized starch comprises from 5 to 50 percent by weight on a dry solids basis of the total adhesive composition with from 10 to 40 percent being preferred and 12 to 30 percent being more preferred.
The peelable pressure sensitive adhesives of the invention optionally further comprise one or more additional ingredients including starch copolymer compositions as well as tackifying resins, surfactants, defoamers, rheology modifiers, antioxidants, biocides, fungicides, fragrances and the like, in specified proportions to produce aqueous dispersions that achieve the desired adhesive properties. According to one aspect of the invention the adhesive composition comprises up to 35 percent by weight of tackifier resins. According to another preferred aspect of the invention the adhesive composition comprises up to 2.0 percent by weight of one or more surfactants. Preferred adhesive compositions of the invention further comprise up to 1.0 percent by weight of defoamers and optionally up to 4 percent by weight of a crosslinker.
While a variety of adhesive types can be prepared by this approach, for example, remoistenable adhesives, the invention is particularly directed toward pressure sensitive adhesives. The exact properties of the resulting adhesive product can be controlled by the selection of the type of components and their ratio.